For determining the level of contents in a container, measuring systems are used that measure different physical variables. On the basis of these variables, the desired information about the level is then derived. Besides mechanical scanners, capacitive, conductive or hydrostatic measuring probes are used, as are detectors that operate on the basis of ultrasound, microwaves, or radioactive radiation.
In many fields of use, such as petrochemicals, chemistry and the food industry, highly accurate measurements of the level of liquids or bulk goods in containers (tanks, silos, etc.) are needed. Increasingly, sensors are therefore used in which brief electromagnetic high-frequency pulses (TDR methods or pulse-radar methods) or continuous frequency-modulated microwaves (such as FMCW-radar methods) are input into a conductive element or a waveguide and carried into the container where the contents are stored by means of the waveguide. The known variants can be considered as the waveguide: surface waveguides on the Sommerfeld or Goubau principle, or Lecher waveguides.
In physical terms, in this measuring method, the effect is exploited that at the boundary face between two different media, such as air and oil or air and water, some of the guided high-frequency pulses or the guided microwaves carried are reflected because of the abrupt change (discontinuity) in the dielectric constants of both media and is returned back to a receiver by way of the conductive element. The reflected component (or useful echo signal) is all the greater, the greater the difference between the dielectric constants of the two media. The distance from the surface of the contents can be determined from the transit time of the reflected component of the high-frequency pulses or CW signals (echo signals). If the empty distance of the container is known, then the level of contents in the container can be calculated. If a boundary face determination is to be performed, then the location of the boundary face can be determined from the outcomes of the measurement.
Sensors with guided high-frequency signals (pulses or waves) are distinguished over sensors that freely broadcast high-frequency pulses or waves (free-field microwave systems or FMR, also called “genuine radar systems”) in having a substantially greater echo amplitude. The reason for this is that the power flow is effected quite purposefully along the waveguide or conductive element. Moreover, sensors with guided high-frequency signals have greater measuring sensitivity and measuring accuracy at close range than freely broadcasting sensors.
The measuring accuracy and measuring sensitivity of sensors that use surface or Lecher waveguides is worsened considerably if the transition region from the input unit to the conductive element is located in the region of a container connection stub or—in general terms—in the region of a structural part that is disposed in the container. If that is the case, then there is the risk that the portion of the radiation that is not guided—as desired—in the direction of the surface of the contents but instead is broadcast toward the side will lead to transverse resonances (or in the case of a connection stub, to void resonances). Moreover, because of the surface waves along the conductive element, longitudinal resonances can develop. The interfering echo signals that this causes can become so strong that the actual useful echo signal is no longer detectable. Moreover, if longitudinal resonances occur from reflection in the propagation direction, the attenuation of the amplitude of the surface wave and hence of the useful echo signal is especially problematic.
One problem that occurs in particular—but not exclusively—when the sensor is secured in the connection stub of a container is the development of deposits. These occur especially in containers that are filled with hot media or in containers located outdoors that are exposed to major temperature fluctuations. When dust additionally develops in the container, a deposit then forms that can grow over time to such an extent that the transmission of the surface waves is entirely suppressed, or that at least interfering echo signals at close range are created.